Samples of plastic waste level in ocean water taken in the North Pacific.
Photograph: Getty Images

Scientists believe they have solved the mystery of where tens of thousands of tonnes of missing tiny pieces of plastic are ending up – and the answer lies in the mud and sand on the ocean floor.

Researchers have previously been puzzled by why they found much less plastic on the ocean surface than they expected, but a study by a British and Spanish team concludes that deep sea sediments are acting as a sink for such “microplastics”.

Analysing samples from 12 sites in the Atlantic Ocean, Mediterranean Sea and Indian Ocean taken between 2001 and 2012, they found for the first time that substantial quantities of microplastics – which measure less than 1mm in length – had accumulated in deep sea sediment.

The tiny fibres were found at depths from 300m down at the shallowest in the Mediterranean to over 3,000m deep, at volumes 1,000 times higher than those at the surface of the sea.

Prof Lucy Woodall, of the Natural History Museum in London and the paper’s lead author, said: “This is the tip of the iceberg. Fibres are ubiquitous in our oceans and they do appear to be quite abundant in comparison with similar studies that have looked at similar things. The fundamental message of the paper is really quite simple: they’re there. Now we need to find out what the impacts are on our environment.”

A study earlier this month, the most comprehensive of its kind so far, estimated there are more than 5tn pieces of plastic in the world’s oceans, weighing nearly 269,000 tonnes. But the authors, who collected tens of thousands of pieces of plastic and then extrapolated that to model how many would be found worldwide, cautioned that the amount was just 0.1% of annual global plastic production.

“The prevalence of plastic microfibres in all sediment cores and on all coral colonies examined suggests this contaminant is ubiquitous in the deep sea. Furthermore, the wide variety of polymer types detected reveals that the accumulation and deposition of microfibres in the deep sea is complex and that they arise from a variety of domestic and industrial sources,” the study said.

Woodall added: “Pretty much everything [is a potential source for what we found]. Just look around in our environment, our computers have plastic, our bags have, our cups have. All those things can potentially end up in the ocean, so to pinpoint any particular source is just not possible.”

The abundance of plastic at such depths has potentially negative ramifications for marine life, though the study says more research is needed. “A range of organisms are known to ingest microplastics, and there is concern this could result in physical and/or toxicological harm,” the authors warn.